Cycling is a really good low impact form of exercise that gets you out into the fresh air and builds your cardiovascular and muscular endurance. Why not download Strava to track your rides to see how far and fast you are going and you can choose to share with your friends if you want to get competitive.
Here are some ideas if you want a target to work to. Remember to get improvement it is best to do at least 2 or 3 rides per week.
New to cycling:
5K ride: aim to complete this distance in under 30 mins
10K ride: aim to complete this distance in under 35 mins
20K ride: aim to complete this in under 50 mins
25K ride: aim to complete this in under 50 mins
Be aware that these times will be harder to achieve on mountain bikes and on off road trails!